Return variable length array from function

Question

Been in C#-land for a while and I can't work out how to do this in C++ (in an Arduino sketch)

I would like to call a function from a library that returns a list of bytes of unknown length. Sort of like this:

byte devices[] = MyLib::EnumerateDevices();

And in the library:

byte[] MyLib::EnumerateDevices()
{       
   int count = 0;       

   //some code that modifies count

   static byte *temp = new byte[count];  // Assume count is 2 here

   temp[0] = 42;
   temp[1] = 44;       

   return temp;
}

Obviously I have all me pointers and derefs either missing or in the wrong place...

Help?

Dave

You'll need some way to pass the length of the array as well, unless you can deduce the length just from the byte sequence? — Drew McGowen
– Drew McGowen, Commented Jul 24, 2013 at 14:37
The arduino uses AVR libc which does not support new and delete: nongnu.org/avr-libc/user-manual/FAQ.html#faq_cplusplus — Craig
– Craig, Commented Jul 24, 2013 at 16:08

user529758 · Accepted Answer · 2013-07-24 14:39:33Z

7

This is what vectors are for:

std::vector<int> func()
{
    std::vector<int> r;
    r.push_back(42);
    r.push_back(1337);
    return r;
}

Vectors have a size() member function which returns exactly what you want.

If you want a pointer out of the vector, then write

const int *p = &vec[0];

(obviously, substitute int with whatever type you specialized the vector with.)

answered Jul 24, 2013 at 14:39

user529758

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5 Comments

Mike Seymour Over a year ago

These days, you could shorten that to return {42, 1337};, if you like brevity.

user529758 Over a year ago

@MikeSeymour well, yes. My go was only to show that "here you do stuff with the vector and then return it" :)

Craig Over a year ago

std::vector (and the rest of the STL) is not available on arduino.

user529758 Over a year ago

@Craig Then you need to pass a size_t argument by reference.

Lapys Over a year ago

Sorry, a bit pissed. The question specified variable-length arrays in C++, NOT vectors! (Feel free to delete this comment now...)

Mats Petersson · Accepted Answer · 2013-07-24 14:40:36Z

2

You can't return an array in C or C++. You can return a pointer, but in this case, you would also need to return the size. Use std::vector<int> instead, much easier.

answered Jul 24, 2013 at 14:40

Mats Petersson

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1 Comment

user1095108 Over a year ago

You can return an array, either as a struct/class member or a std::array<>.

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